Technical Field
The present invention relates to a method for evaluating cardiac function, in particular a method that utilises the information provided by electrocardiography. The invention also relates to an apparatus in which the aforementioned method may be practised, including a computer program.
History of Related Art
The Intrinsic conducting system of the heart permits electrical impulses originating from the sinoatrial node to travel through the cardiac tissue in a controlled manner. The passage of this electrical impulse through the heart tissue produces a wave of contraction through the cardiac tissue. The wave of contraction is followed by a period of relative electrical calm in the heart tissue, which corresponds to relaxation of the cardiac tissue. Arrhythmias occur when this normal, organised electrical activity of the heart becomes disrupted. Worldwide 3 million people a year die from sudden cardiac death. In most cases there is no warning and the heart is stopped by a sudden arrhythmia. Some people are at high risk of sudden cardiac death, but this can be prevented by an implantable cardioverter defibrillator, which is implanted in a minor operation.
In the UK, subjects are screened for risk of sudden cardiac death using the National Institute for Health and Clinical Excellence (NICE) guidelines (a screening that is based on a mixture of physiological and electro-physiological measurements and an understanding of the subject's clinical history). However, most of the people who die from sudden cardiac death are not identified by these guidelines.
Assessment of the health of the heart by measuring its electrical activity is known. For example, one can measure the electrical activity of the heart with the use of intra-cardiac electrodes that are directly applied to the cardiac tissue.
This is however a particularly invasive technique that is not preferable for the routine assessment of subjects and that has not been clearly shown to demonstrate any clinical relevance for assessing cardiac function such as the risk of arrhythmia. Electrocardiography (ECG) has been developed as a non-invasive procedure for studying the electrical activity of the heart. ECG involves the placing of a plurality of electrodes on the skin surface of a subject. An understanding of the electrical activity of the heart may be identified from the potential difference (i.e. leads) between combinations of the plurality of electrodes. Conventionally, a collective assessment of ECG leads provide a classic ECG tracing, which comprises a P wave, a QRS complex and a T wave, and which demonstrate periods of electrical activity that vary from the isoelectric line. It has been suggested that ECGs may be useful for identifying arrhythmia of the heart by measuring the dispersion of QT durations on an ECG tracing. Measuring changes in this QT duration as an indicator of cardiac arrhythmia has however since been discredited; to the degree that the cardiology community no longer view the QT dispersion assessment as a clinically relevant way to establish arrhythmia risk (see, for example, Malik et al: JACC; 2000:36:1749-66).